The invention relates to a scanner device for measuring the color properties of a measured object pixel by pixel by means of a color measuring head of the type with a support surface for the measured object and a drive unit for moving the color measuring head above the support surface in at least one dimension thereof and for adjusting the height of the color measuring head in the direction perpendicular to the support surface, and having a measurement and drive control unit which activates the drive unit and co-operates with the color measuring head.
Published international patent application WO 2006/045621 A1 provides a detailed description of an automated measuring system for quality control and for controlling the color of a printing machine. It essentially comprises a measuring table on which a printed sheet to be measured can be placed, and a measuring unit comprising a beam-shaped line scanner and an individual color measuring head. The line scanner and individual measuring head can be moved above the measuring table by a computer-controlled drive mechanism so that it travels across every pixel of the printed sheet to be measured. The quality and control parameters needed for the printing process are derived from the resultant measurement values. The output parameters generally include color measurement values CIE XYZ and derived calorimetric variables (in accordance with CIE publication 15 and ISO 13655) as well as density measurement values in accordance with ISO 5. The individual measuring head is advantageously provided in the form of a spectral measuring head so that all the output parameters can be computed in a known manner from the measured spectral values of the reflection factor. As a rule, the individual measuring head is used to measure print control strips contained in the printed sheet, whilst the line scanner is used to scan the rest of the printed image content.
The size of the measurement field in the print control strip is tending to become increasingly small, on the one hand in order to save on waste and on the other hand to enable more measurement fields to be provided in the print control strip and control more printing systems with one strip.
Measuring small measurement fields cleanly at a high scanning speed requires a more complex design of measuring optics than is available in the prior art. Parameters such as the sensitivity to distance and positioning accuracy of the measuring head must satisfy significantly higher requirements. Allowance must also be made for the homogeneity of the measurement fields if working with small measuring orifices.
Since it is necessary to measure freshly printed print samples, it is important to have a geometry which hovers freely. The design of the measuring system must be such that it can be adapted to different paper thickness and take account of fluctuations in the planarity of the sample bed.
To obtain highly accurate measurements, it is extremely important that the peripheral distance between the illuminating and measuring speckles of the color measuring head on the measured object is kept as small as possible. In the case of the 45°/0° measuring geometry used as standard, the size of the peripheral distance depends on the sum of all variances occurring in the z direction (height variances) plus adjustment errors, and contains the maximum planarity errors of the measured sample across the entire measured surface, the positioning accuracy of the capture speckle compared with the illuminating speckles and the lack of imaging sharpness in the region of the edges of the illuminating speckle and capture speckle.
The AxisControl measuring system by the Heidelberg company uses a compact spectral measuring head with a two-dimensional drive across a planar sample sheet surface. The measuring head can not hover freely during measuring. The distance is controlled by mechanically placing the measuring head on the sample. The position of the control strip on the sheet must be manually determined with a light pen. The measuring technology is designed for spectral measurements without polarization filters only and typically requires a measuring field size of 5 mm×6 mm.
The Intellitrax measuring system by X-Rite Inc. combines a spectral color measuring head with a tracking sensor, which enables automatic tracking and positioning relative to the print control strip. The optical measuring system is designed so that it operates at a fixed distance from the test sample and must therefore tolerate the entire range of different paper thicknesses. When using standard 45°/0° geometry, the distance tolerances require there to be a correspondingly high peripheral distance between the illuminated and captured surface in the measurement field. For a broad range of paper thicknesses from 0 to in excess of 1 mm, this system restricts the size of the minimum measuring speckle and hence its use for detecting the smallest measurement fields without crosstalk.
Another measuring system is described in document DE 195 30 185 C2. In this known system, the problem of the distance is solved by creating an air cushion between the measuring head bottom edge and the paper surface, on which the measuring head hovers. A constant working distance is obtained by the air cushion irrespective of the paper thickness.